Support Python 3.10

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = docs,flake8,mypy,twine,coverage,py{36,37,38,39},du{14,15,16,17}
+envlist = docs,flake8,mypy,twine,coverage,py{36,37,38,39,310},du{14,15,16,17}
 
 [testenv]
 usedevelop = True
@@ -15,7 +15,7 @@ passenv =
     EPUBCHECK_PATH
     TERM
 description =
-    py{36,37,38,39}: Run unit tests against {envname}.
+    py{36,37,38,39,310}: Run unit tests against {envname}.
     du{12,13,14}: Run unit tests with the given version of docutils.
 deps =
     du14: docutils==0.14.*